Check full proxy chains in QUIC network transaction tests + clean-up

This CL updates several QUIC network transaction tests so that the
full proxy chain is checked instead of only checking the scheme of
the first proxy server. This will enable tests of multi-proxy chains
for IP Protection in the future. Decoupling the helper methods from
the type of proxy chain used also allows more tests to use them.

Also, this CL has some other no-op changes / clean-up:
 - Adds a new SendRequestAndExpectSpdyResponseFromProxy method
 - Refactors response data into const variables when it’s used in
   a check later. A convention seems to be that "hello!" is used
   for QUIC response data and "hello world" is used for HTTP
   response data, so this has been preserved in the naming of the
   variables.
 - Updates naming of several other const variables
 - Removes some unneeded code / comments

Bug: 1491092
Change-Id: Ic74efcb35ae4f4035b2492d29a6eca4c2b9b725c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5245174
Reviewed-by: Bence Béky <bnc@chromium.org>
Commit-Queue: Andrew Williams <awillia@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1254684}

• net/quic/quic_network_transaction_unittest.cc[diff]